There are known in the prior art electrostatic copying machines in which a latent electrostatic image produced on the surface of a photoconductor is developed by subjecting the image to the action of a developer made up of a mixture of ferromagnetic particles and of fusible toner particles. In general there are two systems for applying the developer to the surface of the photoconductor carrying the latent image. In the first of these arrangements a conveyor is employed to carry developer from a supply to a location at which it is dumped onto the photoconductive surface so as to cascade over the image. As the developer moves over the image, the triboelectric toner particles adhere to those areas of the image which have retained charge after exposure. The cascaded material leaving the photoconductive surface falls under the influence of gravity back into the supply of developer.
In the other widely used dry developer applicator system, an arrangement of permanent magnets disposed within a hollow rotating cylinder of magnetic material causes ferromagnetic particles of developer in which the cylinder is partially immersed to be attracted to the surface of the rotating cylinder in such a way as to form what is known as a "magnetic brush." As the brush cylinders rotates, the brush engages the surface of the drum or the like carrying the photoconductor, and toner particles in the developer mixture on the brush adhere to the drum surface in areas thereof which have retained charge following exposure to an image of the original to be copied. The arrangement of the permanent magnets is such that developer is permitted to fall from the surface of the magnetic brush drum after it leaves the surface of the photoconductor.
As images are developed in an electrostatic copier, toner from the developer mixture is depleted, while the ferromagnetic carrier particle content of the developer remains the same. In order that faithful copies be made with the desired degree of consistency, it is necessary that the proportion of toner to carrier, or the toner concentration, remain substantially constant. In attempts to achieve relatively constant toner concentration in machines of the prior art, a number of approaches have been taken. The first and most elementary of these is periodic actuation of a toner replenisher supply to add the toner constituent to the developer mixture periodically. This arrangement has not proven satisfactory for the reason that it does not take into account variations in the proportion of dark to light areas in documents or the like being copied. That is to say, over one period of time documents having large black areas requiring relatively great amounts of toner may be copied, while during another period of time documents having great white areas requiring very little toner may be copied. Where toner is periodically added to the developer mix, there may be either too much toner in the mix or too little at any one time, resulting in unsatisfactory operation.
Various proposals have been advanced for overcoming the problem outlined above resulting from periodic addition of toner to the developer supply. All of these attempts involve monitoring the developer to obtain a measure of the concentration of toner therein.
Shelffo et al U.S. Pat. No. 3,527,651 shows an arrangement for use in a developer applicator system of the magnetic brush type, in which an electrode is positioned in contact with the bristles of the brush in an attempt to measure changes in the electrical resistance of the developer. The difficulties inherent in such a system will readily be apparent. There is, first, the uncertainty of contact with the bristles of the brush and the criticality of the position of the electrode with relation to the brush cylinder. Further, owing to the wide variation in size of the carrier particles, the brush structure is irregular, adding to the uncertainty of the measurement.
Gawron U.S. Pat. No. 3,707,134 discloses an arrangement in which an attempt is made to determine toner concentration by positioning an iron core coil adjacent to the magnetic brush cylinder with the coil axis generally perpendicular to the cylinder axis. Permanent magnets are employed to attract the carrier particles on the cylinder toward the end of the coil. Changes in the inductance of the coil in response to changes in toner concentration on the portion of the developer adjacent to the coil are used to add toner to the developer mix. This arrangement likewise is relatively uncertain, since only a very small amount of developer is being monitored at any instant. In addition, it is relatively critical, since, as the patentee points out, the magnets employed, while they must be of sufficient strength to attract particles toward the end of the coil, must not be sufficiently strong to cause the particles to move onto the end of the coil or a false indication will be obtained.
Furuichi U.S. Pat. No. 3,698,926 discloses a developer monitoring arrangement for use with a developing system of the cascade type. In the arrangement shown in this patent, developer material falling off the photoconductive surface is collected in a funnel of nonmagnetic material having a cylindrical outlet tube surrounded by a coil. While the patentee suggests that the bottom opening of the funnel be made relatively narrow, he discloses no means for controlling the flow of developer from the bottom opening of the funnel through the cylindrical tube carrying the coil. We have found that, while passing developer material through a coil represents a good approach to the problem of monitoring toner concentration, the results achieved, however, are greatly affected by the manner in which the material flows through the coil. More particularly, we have discovered that if the material is permitted to pass freely through the funnel discharge tube as in Furuichi, the results produced are unreliable.
We have invented a toner concentration detector and replenisher which is especially adapted for use with a magnetic brush developing system. Our detector produces consistently reliable results. It is certain in operation. It insures that the toner concentration remains substantially constant. Our detector accurately measures relatively small changes in toner concentration.